area of triangle given the length of sides is given by
\[\sqrt{s(s-a)(s-b)(s-c}) \]
where s is semiperimeter of triangle and a ,b ,c are the length of sides of triangles ABC
\[2s=a+b+c \]
For a= 3 b=4 and c=5
\[s-a=3 \]
\[ s-b = 2 \]
\[ s-c =1 \]
hence area=6